There are many potential uses for coherent electromagnetic radiation sources. While there are uses for electromagnetic radiation at various frequencies, there exists particular promise for sources generating waves in the terahertz (THz) frequency range, generally identified as the electromagnetic spectral range above microwave frequencies and below the far infrared frequencies.
Two main areas of interest in the use of terahertz waves are in the fields of imaging and spectroscopy. Imaging is useful for many applications. However, presently, the dominant market demand on imaging is for security applications. Being at such a low frequency (compared to X-ray) THz waves are significantly safer to use as they do not ionize tissue. However low power and/or low frequency of available sources has limited the resolution. Still the use of THz scanner instead of X-ray scanners has driven this market forward in spite of the lack of convenient and effective sources. Because many materials are transparent to THz waves, such as paper, clothing, semiconductors, and plastics, non-destructive evaluation of such items as the Styrofoam on the space shuttle fuel tank, semiconductor wafers, and wood have been performed. Additional potential imaging uses can include such things as identification of skin cancer, as well use in covert operations to read sealed letters without opening them.
Spectroscopy predominantly has been a research market to date. In spectroscopy, the identification of many compounds, such as the explosives RDX, C-4, and Semtex, has been demonstrated due to the fact that many materials have fundamental rotation and vibration transitions in the THz range. THz radiation has been used to study thin film materials including superconductive devices. Additionally, electrical characterization of semiconductors, showing the dopant levels and purity, has been demonstrated. Water vapor, carbon dioxide, and carbon monoxide content in the exhaust gasses of automotive and jet engines has been studied to determine combustion efficiency. Coherent control of dark states in semiconductor samples has been demonstrated opening another possibility in the development of quantum computing and optical computing.
The ability to do multi-spectral imaging (combining the two) is a recent promising development of broadband sources. However, known optical sources are of very low spectral brightness and therefore are very limited in methods of detection. However, multi-spectral imaging systems have demonstrated the ability to recognize the difference between several different drugs hidden in a suitcase without opening the case. Pharmaceutical companies can employ multi-spectral THz imaging to inspect the quantity and purity of their products as quality control.
There are many additional potential uses for THz radiation, such as studying plasmas in experimental fusion reactor devices, identifying droughts by non-destructively examining the water content in a tree's wood, and even non-destructively examining paintings for hidden underlying works. Unfortunately, many potential uses have had their development or implementation prohibited due to the lack of suitable sources. While there are known terahertz sources in existence, these existing sources are bulky, have inconvenient operating parameters, or are limited in their output power and/or frequency range (e.g., many cannot reach above 2.2 THz, etc., as but one example of an applicable present limitation on known sources).
Previously, the production of terahertz waves has proven to be difficult, as the terahertz spectral range is above microwave in frequency, and thus too high for conventional electronic methods, while being very low in the far infrared range, making the frequencies difficult to generate via optical techniques. In electronic devices, the scale of the physical components is very small, making accurate construction difficult for devices such as backwards wave oscillators (BWOs)1, traveling wave tubes, Gunn diodes, and Shottky diodes2 used as multipliers fed by either microwave sources or BWOs. While it is possible to build a gas laser to produce discrete lines in the THz range, such devices are bulky as the required low pressure, strict temperature control, and long interaction path length from the pump laser limit how small such devices can be built3. In semiconductor laser devices the energy equivalent to photons in the THz range is in the milli-electron-volt range (1 THz˜4.3 meV) and the equivalent temperature to 1 THz is 48° K. Therefore, a semiconductor device, in which the charge carriers drop in voltage in steps, known as a “quantum cascade laser” (QCL)4, uses cryogenic cooling to allow fixed frequency device construction. As an alternative approach, optical parametric oscillators (OPOs) performing difference frequency generation between the signal and idler in a non-centro-symmetric crystal, such as gallium arsenide, can be used to build a tunable source which is table top sized; but, such source requires precise alignment and constant adjustments. Unfortunately, this requires significant operating expertise and considerable effort on the part of the user5. Additionally, there are two types of broadband THz sources both based on nonlinear interactions using ultra-short pulses in the near or mid-IR range to generate THz in non-centro-symmetric crystals or photoconductive switches. 1 W. C. Hurlbut, V. G. Kozlov, “Extended spectral coverage of BWO combined with frequency multipliers”, conf. proceedings Photonics West, 2010.2 T. W. Crowe, J. L. Hesler, R. A. Retslov, et. al., “Solid state LO sources for greater than 2 THz”, 2011 ISSTT digest, 22nd symposium on space terahertz technology, 2011.3 E. R. Mueller, “Optically-pumped THz laser technology”, Coherent lasers Inc.4 B. S. Williams, “Terahertz quantum-cascade lasers”, Nature Photonics, Vol. 1, September 20075 P. F. Tekavec, W. C Hurlbut, V. G. Kozlov, et. al., “Terahertz generation from quasi-phase matched gallium arsenide using a type-II ring cavity optical parametric oscillator”, Cong. Proceedings Photonics West, 2011.
While some electronic terahertz-generation devices have been developed in recent years, few are narrow band and tunable. Of the few that are narrow band and tunable, they tend to group into three classes: electronic, semiconductor opto-electronic, and nonlinear optical sources. With the electronic sources, invariably as one goes to higher frequency generation, the construction of a suitable device becomes more and more difficult and power levels drop to microwatt levels by 1.5 THz. Producing power below 4.3 THz using QCLs (the second category) requires liquid nitrogen or colder cryogenic cooling as the thermal vibration of the cavity degrades the resonance of the cavity to the point there is no resonance to establish the cascade effect if the temperature is not reduced below the equivalent temperature of the THz photons to be generated. While QCLs can generate mW power levels, the cryogenic requirements make them difficult to work with and commercially these are not feasible below 1 THz. Nor are they tunable. While the third category can run at room temperature, can be tuned, and systems with up to 2.9 mW output have been built, the alignment and operational difficulties along with the required stability (e.g., suspended optical tables, etc.) to avoid vibration, which reduces or eliminates power output and can change the frequency (mode hopping) make these systems impractical for industrial and security applications.
It is worth noting that terahertz waves see significant attenuation due to water vapor absorption across most of the spectral range. There are, however, known windows where the atmospheric absorption due to water vapor is minimized. These occur at 0.65, 0.85 and 1.5 THz6. Therefore, these frequencies are desired operating frequencies for fixed frequency long range THz devices. Fixed frequency devices such as Gunn diodes can be constructed for the lower two of these frequency windows with mW power levels. However, the upper one is out of bounds of fixed electronic devices. QCLs can operate at the uppermost window, but, as previously mentioned, require cryogenic cooling and typically producing 10 s of microwatts of power. 6 L. S. Rothman, HITRAN database, Atomic and molecular physics division, Harvard-Smithsonian center for astrophysics, 2008.
Among known tunable devices, BWOs and Schottky diode chains on high power microwave sources can operate at the lower frequency windows. However, the more desirable (due to the shorter wavelength) frequency of 1.5 THz is beyond the range of BWOs. Additionally, it is a range where Schottky diode devices make only 10 s of microwatts of power. While OPOs can operate in any of these ranges, the typical power is less than 1 mW, and they exhibit previously mentioned alignment and stability disadvantages.
More recently, hot microbolometer cameras and pin diode cameras have become available in the THz range from vendors such as NEC7, INO8, Traycer9. Disadvantageously, these cameras typically have noise equivalent power (NEP) between 10 and 100 pW per pixel element. Therefore, to conduct imaging with a 1000 pixel-element camera and have a minimum of 100:1 contrast ratio would require a source of 10 mW average power.
While it has been known since the 19th century10 that RLC electrical circuits will oscillate in a well-known manner based on the values of the resistance, R, capacitance, C, and the inductance, L; use of said devices as a source has largely been ignored because the disparity between wavelength and device size coupled with the impedance mismatch to free space result in very low coupling of electrical power to electromagnetic radiation at frequencies into the microwave spectral range. In 1996 it was discovered that split-ring resonators, a specific type of RLC circuit, make good antenna arrays in the microwave and higher frequency regimes11. These arrays are commonly called meta-materials as the split-ring resonator is considered a “meta-particle” with an engineered absorption spectrum. When placed into a periodic two dimensional array, this lattice structure is referred to as a 2-D meta-material. Such meta-materials are known to act as strong absorbers when designed and operated in the terahertz frequency range (between 100 GHz and 10,000 GHz).12 However, an antenna that has strong reception capacity also generally presents a good design for a broadcast antenna. This is due to the requisite strong coupling between the free space wave propagation and the electrical signal, which is an identical process of impedance matching and scaling for both transmission and reception. 7 N. Oda, M. Sano, K. Sonada, et. al., “Development of terahertz focal plane arrays and handy camera”, Conf. proceedings SPIE Infrared technology and applications, Apr. 25, 2011.8 M. Bolduc, M Terroux, B. Tremblay, et. al., “Noise-equivalent power characterization of an uncooled microbolometer-based THz imaging camera”, Conf. proceedings SPIE9 H. L. Mossbacker, J. Alverbro, Z. Zhang, et. al., “Initial test results for a real-time 80×64 pixel, 600 GHz-1.2 THz imager”, Conf. proceedings IRMMW-THz 2011, Oct. 2-7, 2011.10 J. C. Maxwell, A Treatise on Electricity and Magnetism, Dover (1904).11 Pendry, J. B.; Holden, A. J.; Robbins, D. J.; Stewart, W. J., “Magnetism from conductors and enhanced nonlinear phenomena”. IEEE Transactions on Microwave Theory and Techniques 47 (11): 2075.12 B. Jitha, C. S. Nimisha, C. K. Aanandan, P. Mohanan, K. Vasudevan, “SRR loaded waveguide band rejection filter with adjustable bandwidth”, Microwave Opt. Technol. Lett. 48: 1427-1429, 2006.
Having recognized this, scientists around the world have attempted to develop an active meta-material device13. The main efforts have been to use a laser source to nonlinearly couple energy so that an incoming signal is amplified. The reason for this seemingly less direct approach is due to several issues for standard semiconductor methods of electrical stimulation, such as the high capacitance that electrically coupling these devices would provide, effectively lowering the spectral operating range below the optimal coupling range of the device, and the THz absorption that would result from the presence of highly doped semiconductors inside the meta-material14. 13 H.-T. Chen, W. J. Padilla, J. M. O. Zide, et. Al., “Active terahertz metamaterial devices,” Nature 444, 597 (2006).14 H.-T. Chen, A. K. Azad, J. F. O'Hara, et. Al., “Active Terahertz Metamaterials”, Conf. Proceedings IRMMW-THz 2011, Tu-1.2, 2011.
While solid state sources in the THz, far-infrared, and mid-infrared ranges do exist in the form of quantum cascade lasers, they are limited to a very narrow tuning range and require cryogenic cooling15. In the wavelength range shorter than approximately 5 μm through the visible range, semiconductor lasers have been developed; but, they also are of limited tuning range16. 15 B. S. Williams, “Terahertz quantum cascade lasers”, Nature Photonics, vol. 1, September 2007.16 P. Zorabedian, F. J. Duarte., Tunable Lasers Handbook, Academic Press, 1995.